When designing sample measuring instruments which operate in a bath of liquid helium, it is common to provide for the cooling of the sample by drawing some of the liquid up from the bath into the region of the sample. The liquid is drawn by a pressure difference through a small diameter capillary tube into an insulated chamber in which the sample is mounted. If temperatures below approximately 4.2K are required, the chamber is evacuated to a pressure such that the liquid helium in the chamber boils at that temperature. If temperatures above 4.2K are required, a heater is used which boils the liquid and heats the vapor to the desired value. Through a combination of these techniques, a range of temperatures from about 2.degree. K. to above room temperature may be achieved.
There are, however, several serious difficulties with this straightforward temperature control scheme. The capillary tube must be made large enough that a significant flow of helium can be obtained. This is necessary so that the sample chamber can be cooled in a reasonable time period and to provide responsive temperature control in general. This large capillary, however, makes it difficult to achieve temperatures well below 4.2.degree. K. when pumping strongly on the liquid helium in the chamber. The reason is that the reduced vapor pressure above the bath, in addition to cooling the helium already in the chamber, also pulls more 4.2.degree. K. liquid into the chamber at a high rate. This higher temperature helium creates a large heat load on the chamber and limits the ultimate low temperature of the instrument.
One technique which has been used to avoid this problem is to provide a mechanical valve at the inlet of the capillary tube. In this way, it is possible to use a large capillary to allow rapid cooling and to admit a quantity of liquid into the sample chamber. The valve can then be closed so that no further liquid enters the chamber while this quantity is cooled by evacuation. When the liquid in the chamber is exhausted, the process must be repeated. Unfortunately, the difficulty of making reliable cryogenic valves has limited the commercial usefulness of this approach.
A second problem occurs in the temperature region between about 5.degree. K. and 20.degree. K. The liquid helium which is being drawn through the capillary will be vaporized before it reaches the chamber or just as it enters the chamber. If the helium is being vaporized in the capillary, before it reaches the chamber, then increasing the amount of heat applied to the bottom of the chamber will increase the temperature of the chamber; this is the expected behavior. However, if the helium liquid is in the chamber, then increasing the power applied to the heater may actually cause the chamber to cool. This is because the heat will cause a rapid flow of freshly vaporized 4.degree. K. gas through the chamber. Any feedback control system implemented to regulate the temperature of the chamber will respond by increasing the power to the heater, vaporizing more liquid and cooling the chamber even further. At some point, all the liquid in the chamber will be vaporized and the chamber will heat up well above the desired temperature.
The system just described is similar to a relaxation oscillator, and is caused be the presence of the two helium phases in the chamber region. Above about 20.degree. K. the oscillations cease to be a problem due to the increased heat capacity of the chamber relative to the heat capacity of the helium gas. This has a damping effect on the system. The higher temperatures in the chamber also tend to keep the liquid-gas interface pushed down into the capillary, and less liquid is available to participate in the process.
It should be appreciated that the cryogenic valve mentioned earlier in this section would not alleviate this oscillation problem.
Finally, the presence of liquid helium or dense helium gas in contact with the sample can cause errors in certain types of measurements. Its presence also makes the insertion and removal of samples difficult when the chamber is at cryogenic temperatures.